1. Field of the Invention
The present invention relates to the improvement of a control method for a PWM converter.
2. Description of the Prior Art
A Japanese patent publication No. 37864/86 is an example of a control device which controls reactive power in an AC/DC converter. This is a control device for an AC motor using a voltage type pulse-width-modulation inverter, wherein reactive power at the side of an AC power supply and a DC output voltage of an AC/DC converter are detected, and the magnitude of an inphase component voltage (hereinafter referred to as an inphase voltage) with respect to the power supply voltage is controlled so as to regurate the reactive power to a prescribed value, at the same time the magnitude of a component voltage having a phase of 90 degrees different from that of the power supply voltage (hereinafter referred to as an orthogonal voltage) is controlled so as to regurate a DC voltage to a prescribed value. In this way, motor operation and regenerative breaking of a motor are made possible, and also the power factor of the AC power supply can be improved.
In PWM inverters, the one for improving output voltage fluctuations during voltage discontrolled intervals caused by an inductive load is described, for example, in a Japanese patent, laid open No. 229676/85.
There has been a point of issue in a conventional voltage type PWM AC/DC converter constituted as mentioned in the above that when a power supply voltage is ascending, especially when a regenerative current is increasing, a set value Vd* of an orthogonal voltage and a set value Vq* of an inphase voltage become large as described later, because of this a voltage instruction-value V* (for each phase of AC three-phase voltage, Vu*, Vv* or Vw* respectively) becomes large.
FIG. 1 is a vector diagram for explaining the above-mentioned point. In the figure, let E be a power supply voltage, let Iq be an active current and if a reactive current Id=0, the output current I is equal to the active current Iq and its phase is inphase with the power supply voltage E. The voltage drop through a reactor (in FIG. 2 described later, it corresponds to a part of a reference number 2) is .omega.LI which is inserted between the AC power supply (in the same figure it corresponds to a part of a reference number 1) and a power converter (in the same figure it corresponds to a part of a reference number 3). Therefore as shown in the figure, the voltage instruction value V* is expressed in the equation V*=E-.omega.LI, and Vd* and Vq* are components of the voltage instruction-value V* respectively, Vd* is an orthogonal-voltage set-value with a phase making a right angle with the phase of the power supply voltage E, and Vq* is an inphase-voltage set-value being inphase with the power supply voltage E.
As clearly shown in the figure, under the above-mentioned condition with a good power factor, the voltage instruction-value V* becomes very large. A utilizable three-phase AC voltage is limited by a DC voltage Ed (in FIG. 2 described later, it corresponds to a detected voltage by a voltage detector 5), so that there is a problem that the control is apt to be plunged into unstableness caused by voltage saturation.
When an enough large voltage margin is given to the voltage saturation by adopting a high DC voltage Ed, the size of a device has to be made large for securing enough insulation.